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Isolation and X-ray Crystallographic Analysis of a Stable Selenenic Acid.

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Isolation and X-ray Crystallographic Analysis
of a Stable Selenenic Acid**
Toshiyuki Saiki, Kei Goto, and Renji Okazaki*
Selenenic acids (RSeOH) play a central role in the oxidation
and reduction processes of organoselenium compounds, for example, in the syn elimination of selenoxides, the reduction of
seleninic acids. and the oxidation of selenols and diselenides.[']
They are also of great interest from a biological point of view as
illustrated by a number of reports that postulate the intermediacy of a selenenic acid in the catalytic cycle of glutathione peroxidase"] and its synthetic model compounds.[3]However, despite
the importance of selenenic acids, much of our knowledge on
their chemistry has been derived in quite an indirect and speculative fashion because of their extremely high instability; they
are known to undergo a rapid disproportionation to form the
corresponding diselenides and seleninic acids. Several areneselenenic acids stabilized by coordination to ~rtho-nitro,'~]
-carbonyl,['] or -amino groups13c1have been reported to be observable, but only in solution.161Even the employment of the
2,4,6-tri-tert-butylphenyl group, one of the most effective steric
protection groups, cannot prevent the disproportionation of a
selenenic acid."] Thus. to date. no selenenic acid has been isolated in pure form,[81and the development of a new methodology
to stabilize this important but elusive species has long been
Herein we report the synthesis and structure of the first
isolable selenenic acid. In this work we have applied the recently
1,4-bridged calas a reaction
ix[6]arene l[9.'01
environment; we had previously
verified the usefulness of 1 as a
stabilizing environment for a
reactive species by incorporating
a sulfenic acid functionality
(X = SOH) into the calixarene 1
as a probe.[gb1
The reaction of p-tert-butyl1
calix[6]arene with selenide 2 under basic conditions afforded a bridged calix[6]arene 3 bearing
a butylselanyl group on the bridge (Scheme 1). Treatment of 3
with benzyI bromide in the presence of NaH gave the tetrabenzyl ether 4 as the main product. Oxidation of 4 by rn-chloroperoxybenzoic acid (rnCPBA) followed by thermolysis in toluene
(80 "C, 2 h) afforded the selenenic acid 5, which was isolated by
silica gel chromatography as colorless crystals (74 %).
Compound 5 was fully characterized by NMR and IR spectroscopy, as well as FAB mass spectrometry, and elemental
analysis. The 77SeNMR spectrum (CDCI,) revealed a signal at
6 = 1134 and the 'H NMR spectrum showed the signal of the
hydroxyl proton at 6 = - 0.05 (readily exchangeable with
D,O), suggesting that it is highly shielded by the calix[6]arene
macrocycle. The 'H NMR spectrum of 5 is consistent with the
cone conformation and hardly changes up to 130°C: it shows
essentially the same features as those in the spectrum of the cone
, L
[*] Prof. Dr. R. Okazaki, Dr. T. Saiki, Dr. K. Goto
Department of Chemistry, Graduate School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113 (Japan)
Fax. Int. code +(3)5800-6899.
e-mail- okazakictr]p
This work was partly supported by Grants-in-Aid for Scientific Research (No.
08454197 and 08740487) from the Ministry of Education, Scienceand Culture,
Japan. We also thank Tosoh Akzo Co., Ltd. for the generous glft of alkyllithium compounds.
Angen Chem Inr Ed l%gl 1997.36, No 20
3 (R = H, X = SenBu)
4 (R = CHPPh, X = SenBt
5 (R = CH?Ph, X = SeOH
Scheme 1. Synthesis of selenenic acid 5. a) KOH, THF/DMF 180%). b) NaH,
PhCH,Br, THF/DMF (59%); c) mCPBA, CH,CI,, then toluene, 80 "C, 2 h (74%)
conformer of its bromide analogue (X = Br instead of
SeOH).[9d1The OH absorption in the IR spectrum (CH,Cl,)
was observed at 3523 cm-', indicating the absence of significant
intramolecular hydrogen bonding.
The structure of 5 was established by X-ray crystallographic
analysis (Figure 1).["I This is the first X-ray structure analysis
Figure 1. Crystal structure of 5 (ORTEP drawing; thermal ellipsoids at 20 % probability level) The aromatic rings of the calix[6]arene macrocycle and that of the
bridging unit are shaded.
of a selenenic acid. The SeOH functionality is deeply buried in
the cavity of thep-tert-butylcalix[6]arene macrocycle with a cone
conformation; evidently this situation is unfavorable for the
intermolecular processes that normally lead to its decomposition. The distance between the selenium atoms on the neighboring molecules is more than 10 A, indicating that there is no
intermolecular interaction between the SeOH functionalities.
The Sel-01 bond length is 1.763(7) A, which is almost the same
as that of the Se-0 single bond in benzeneseleninicacid.["]The
distances between 0 1 and 0 2 - 0 7 lie between 3.88 and 6.21 A,
and are thus much longer than the average distance of hydrogen-bonded HO . . .O (2.72 A),['31clearly showing the lack of
hydrogen bonding between the OH group and the atoms 0 2 0 7 also in the solid state. The Sel-03 distance is 2.64 A, which
is significantly shorter than the sum of the corresponding van
der Waals radii (3.42 A) .[I4] The atoms 0 3 , Sel, and 0 1 adopt
a nearly linear arrangement (bond angle 368"). These structural
parameters indicate the existence of the nonbonding interaction
between the Sel and 0 3 atoms of 5 in the solid state.["]
Selenenic acid 5 is remarkably stable both in the crystalline
state and in solution. Even after 5 had been heated at 120 "C for
5 h in CDCI,CDCl,, no decomposition was observed. Considering that 2,4,6-tert-butylbenzeneselenenicacid disproportion-
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
0570-083319713620-2223$17 SO+ 5010
ates completely within 2 h in 4 % D,O/CD,CN at 25 0C,[71this
stability of 5 is remarkable. In order to clarify its origin, a
control experiment was carried out using selenoxide 6, which
does not have the macrocyclic structure (Scheme 2). Upon heating at 80 "C for 5 min in [DJtoluene, 6 decomposed to afford
diselenide 7 and seleninic acid 8; the intermediary selenenic acid
Ar'SeOH was not detected. After separation, 7 and 8 were isolated in 59 and 28% yields, respectively. These results clearly
indicate that the high stability of 5 is caused by the steric protection, and not the coordination by the ortho substituent.
80 "C, 5 rnin
P h O w O P h
Scheme 2. Control experiment with the selenoxide 6.
The reaction of 5 with mCPBA (1.2 equiv) or 1-butanethiol
(10 equiv) afforded the corresponding seleninic acid 9 (88 Yo)or
selanyl sulfide 10 (78 YO),respectively (Scheme 3), indicating
that 5 retains its reactivity as a selenenic acid although it has the
unprecedented stability.
9 (88%)
10 (78OA)
Scheme 3. Reactions of selenenic acid 5.
In summary, an isolable selenenic acid was synthesized for the
first time by taking advantage of the bridged calix[6]arene
framework, and its structure was determined by X-ray crystallographic analysis. This stable compound should enable us to
elucidate the chemistry of selenenic acids directly.
Experimental Sect ion
To a solution of 4 (282 mg, 0.18 mmol) in CH,CI, ( 5 mL) was added a solution of
mCPBA (80%, 47 mg, 0.22 mmol) in CH,CI, (4 mL) at 0 "C. After the mixture had
been stirred at the same temperature for 2 h, it was washed with 5 % aqueous
NaHCO, and dried over anhydrous MgSO,. After removal of the solvent, the
residue was heated in toluene (9 mL) at 80 "C for 2 h. After removal of the solvent,
the residue was separated by chromatography (SiO,/CHCI,:hexane = 1: 1) to give
5 (204 mg, 74%) in the form ofcolorless crystals. M.p. 168°C (decomp); 'H NMR
(500 MHz, CDCI,): 6 = - 0.05 (s, 1H; SeOH), 1.04 (s, 36H; rBu), 1.44 (s, 18H;
tBu), 3.24(d, ' J = 14.3Hz, 2 H ; ArCH,Ar), 3.34(d, 'J=15.3 Hz , 4H; ArCH,Ar),
4.21 (s, 4H ; ArCH,OAr), 4.47 (d, 'J=11.9Hz, 4H; PhCH,O), 4.500 (d,
'J=14.3Hz, 2 H ; ArCH,Ar), 4.501 (d, 'J=15.3Hz, 4H: ArCH,Ar), 4.58 (d,
'J = 11.9 Hz, 4 H ; PhCH,O), 6.44 (t, ' J = 7.3 Hz, 1 H), 6.81 (d, ' J = 2.3 Hz, 4H),
6.99-7.007(m,8H),7.012(d, 'J=7.3 Hz,2H),7.08-7.11 (m,8H),7.13-7.17(m,
4H), 7.20 (d, "J= 2.3 Hz, 4H), 7.29 (s, 4H); "C NMR (125 MHz, CDCI,):
6 = 26.9 (t), 31.3 (t), 31.5 (q), 31.7 (q). 34.17 (s), 34.19 (s),?2.7 (t), 74.7 (t), 124.2
(d), 124.7 (d), 125.9 (d), 126.0 (d), 127.0 (d), 127.5 (d), 127.9 (d), 128.5 (d), 131.9
(s), 132.1 (s), 133.8 (s), 135.1 (s), 137.7 (s), 137.9 (s), 144.4 (s), 145.6 (s), 152.2 (s),
152.6 (s); 77SeNMR (95 MHz, CDCI,): 6 =1134; IR (CH,CI,): i. = 3523 cm-'
(OH); HR-MS (FAB): calcd. for Ci,~H,,,O,goSe 1530.7730; found: 1530.7737:
calcd for C,,,H,,,O,Se H,O: C 79.09, H 7.55, Se 5.10;
elemental analysis (YO):
found: C 79.31, H 7.53, Se 4.72. The NMR signals were assigned by HH-COSY,
CH-COSY, and differential NOE experiments.
Received: May 5, 1997 [Z10412IE]
German version: Angeu Chem. 1997, 109,2320-2322
Keywords: calixarenes selenium . steric hindrance
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
[I] For reviews, see: a) The Chemistry of Organic Selenium and Tellurium Compounds, Vol. I a n d 2 (Eds.: S . Patai, 2. Rappoport), Wiley, New York, 1986,
and 1987, respectively; b) Orgunoselenium Chemistry (Ed.: D. Liotta), Wiley,
New York, 1987; c) P. D. Magnus in Comprehensrve Organic Chemistry, Vol. 3
(Ed.: D. N. Jones), Pergamon, Oxford, 1979, pp. 491 -538.
[2] a) L. Flohe in Free Rudicals in Biology, Vol. V: (Ed.: W A. Pryor), Academic
Press, New York, 1982, pp. 223-254; b) H. E. Ganter, R. J. Kraus in Methods
Enzymol. 1984, 107, 593-602.
[3] For recent examples, see: a) T. G. Back, B. P. Dyck, J. Am. Chem. Soc. 1997,
119, 2079-2083; b) L. Engman, C. Anderson, R. Morgenstern, I. A. Cotgreave, C. M. Anderson, A. Hallberg, Tetrahedron 1994,50,2929-2938; c) M.
Iwaoka, S. Tomoda, J Am. Chem. SOC.1994,116,2557-2561. and references
[41 a) 0. Behaghel, W. Miiller, Ber. Dtsch. Chem. Ges. 1935,68, 1540-1549; b) H.
Rheinboldt, E. Giesbrecht, Chem. Ber. 1955, 88, 666-678; c) ibid. 1955, 88,
1037-1042; d) &id. 1955,88, 1974-1978.
[5] a) 0. Behaghel, W. Miiller, Ber. Drsch. Chem. Ges. 1934, 67, 105-108; b) W.
Jenny, Helv. Chim. A m . 1958,41, 317-326; c) H. Rheinboldt, E. Giesbrecht,
Chem. Ber. 1956, 89, 631-636; d) H. J. Reich, C. A. Hoeger, W. W. Willis, Jr.,
J. Am. Chem. SOC.1982, 104, 2936-2937.
[6] Although some of those relatively stable selenenic acids were claimed to be
isolable [4,5c]. reinvestigations of the structures of the isolated products revealed that they are not selenenic acids but the corresponding selenenic anhydrides: a) H. J. Reich, W. W Willis, Jr , S. Wollowitz, Tetruhedron Lett. 1982,
23, 3319-3322; b) J. L. Kice, F. McAfee, H. Slebocka-Tilk, ibid. 1982, 23.
(71 H. J. Reich, C. P. Jasperse, J. Org. Chem. 1988, 53, 2389-2390
[8] Possible exceptions are anthraquinoneselenenic acids [5a,b]. whose structural
determination was based only on elemental analysis and UV/Vis spectra.
[9] a) N. Tokitoh, T Saiki, R. Okazaki. J. Chem. SOC.Chem. Commun. 1995,
1899-1900; b) T. Saiki, K. Goto, N . Tokitoh, R. Okazaki, J Org. Chem. 1996,
61,2924-2925: c) T. Saiki, K. Goto, N. Tokitoh, M. Goto, R. Okazaki, Tetrahedron Lett. 1996, 37, 4039-4042; d) T. Saiki, K Goto, R. Okazaki, Chem.
Lett. 1996.993-994. For related compounds, see: e) K. Goto, N. Tokitoh, R.
Okazaki, Angew. Chem. 1995, 107, 1202-1203; Angew. Chem. Int. Ed. Engl.
1995,34, 1124-1126; f) K. Goto, M. Holler, R. Okazaki, J. Am. Chem. SOC.
1997, 119%1460-1461.
[lo] Recently, the bridged calix[6]arenes containing a functional group on the
bridge were also described by Liining et al. a) H. Ross, U. Liining, Angeu.
Chem 1995, 107. 2723-2725; Angew Chem., Int. Ed. Engl. 1995, 34, 25552557; b) Liebigs Ann. 1996,1367- 1373; c) Tetrahedron 1996,52,10879- 10882.
M , = 1530.98, crystal dimensions
[If] Crystal data for 5 : C,,,H,,,O,Se,
0.80 x 0.30 x 0.20 mm, crystal system triclinic, space group P i , unit cell
dimensions u = 12.949(4), b = 28.743(8), c = 12.230(2)A, a = 96.90(2),
8=101.55(2). 7 =78.70(2)", V=4357(1)A3, Z = 2 , p,s,,,=l.167gcm~3,
20,,, = 55.2". The intensity data were collected at 296 K on a Rigaku AFC5R
i0.71069 A), the scan mode being w .
diffractometer with Mo,. radiation (
Of the 20951 reflections which were collected, 20064 were independent. The
linear absorption coefficient p for Mo,, radiation is 4.9 cm-'. An empirical
absorption correction using the program DIFABS 1161 was applied that resulted in transmission factors ranging from 0.81 to 1 .OO. The data were corrected
for Lorenz and polarization effects. The structure was solved by heavy-atom
Patterson methods (PATTY [17]) and expanded by using Fourier techniques
(DIRDIF94 [18]). The non-hydrogen atoms were refined anisotropically. Hydrogen atoms except that of the SeOH group were included hut not refined.
Full-matrix least-squares refinement based on F weighted with l/a2(F,) gave
final values R = 0.063 and W R = 0.066 for 5001 observed reflections
[I> 3.OOo(I)] and 991 variable parameters. Maximumiminimum residual elecCrystallographic data (excluding structure
tron density: 0.81/ - 0.55 e
factors) for the structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication no.
CCDC-100401. Copies of the data can be obtained free of charge on application toThe Director, CCDC, 12 Union Road, Cambridge CB2 lEZ, UK (fax:
int. code + (1223) 336-033; e-mail: deposit(u;
[12] J. H. Bryden, J D. McCullough, Actu Crystallogr. 1954, 7, 833-838.
[13] L. N. Kuleshova, P. M. Zorkii, Acra Crystallogr Sect. B 1981,37,1363-1366
[14] A. Bondi, J. Phys. Chem. 1964,68,441-451.
1151 The intramolecular interaction between a divalent selenium and an amino
group has been studied in detail: a) M. Iwaoka, S. Tomoda, Phosphorus Su@r
Silicon Relat. Elem. 1992,67,125-130; b)J. Am. Chem. SOC.1996,118,80778084. Also see ref [3c]
1161 N . Walker. D. Stuart, Actu Crystuflogr.Sect. A 1983, 39, 158-166.
[17] P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, S. Garcia-Granda,
R. 0. Gould, J. M. M. Smits, C. Smykalla, The DIRDIF program system,
Technical Report of the Crystallography Laboratory, University of Nijmegen,
The Netherlands, 1992.
[18] P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, R. de Gelder, R.
Israel, J. M. M. Smits, The DIRDIF-94 program system, Technical Report of
the Crystallography Laboratory, University of Nijmegen, The Netherlands,
Angeu. Chem. Int. Ed. Engl. 1997, 36, No. 20
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acid, crystallographic, isolation, analysis, selenenic, ray, stable
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